Stainless steel pipe of bright annealing finish type, having highly-smoothed inner surface and method for producing the same

ABSTRACT

A stainless steel pipe of the bright annealing finish type, having a highly-soothed inner surface according to the present invention is characterized in that the inner surface roughness of the pipe, expressed in Rmax is 1.0 micrometer or less, and suitable as a clean pipe for use in an apparatus for the production of semiconductors. According to the method of production of the present invention, a stainless steel pipe having a highly-smoothed inner surface can be obtained by cold drawing and bright annealing treatment only. By this method, such a treatment that has been considered to be essential, for example, electrochemical polishing can be omitted, so that the production cost can be greatly reduced. The stainless steel pipe of the invention can thus be widely utilized in the fields of the production of semiconductors and the like.

TECHNICAL FIELD

The present invention relates to a stainless steel pipe of the brightannealing finish type (BA type), having a highly-smoothed inner surface,suitable as a clean pipe for use in an apparatus for the production ofsemiconductors, and as a method for producing the same.

BACKGROUND ART

Clean pipes are classified, according to the method of the productionthereof, into the bright annealing finish type in which a stainlesssteel pipe after being cold drawn is subjected to bright annealingtreatment; the electric polishing finish type (EP type) in which theinner surface of a stainless steel pipe of a bright annealing finish isfurther smoothed by means of electrochemical polishing; and like method.

It is well known that the inner surface roughness of a clean pipe isclosely related to the production of impurities or fine particles andthe discharge of water vapor from the inner surface of the pipe. In anapparatus which is required to have a high degree of cleanliness, cleanpipes of the electrolytic polishing finish type whose inner surfaceroughness becomes lower and are expensive, are used.

In the production of a pipe having a smooth inner surface, there hasbeen a conventional method employed in which a tubing material issubjected to cold plug drawing. Cold plug drawing is a method ofprocessing in which a tubing material 85 is cold drawn with the outerand inner surfaces thereof constrained, as shown in FIG. 10, by a fixingdie 86 having a round hole and a plug 81, and the outlet-side end of thetubing material 85 chucked (not illustrated). A chemical conversiontreatment lubrication and oil lubrication are the general methods oflubrication between the tools (the die 86 and the plug 81) and thetubing material 85, and oil lubrication capable of forming a thinlubricating film is employed in order to obtain highly-smoothed innerand outer surfaces.

A material for a pipe such as a clean pipe which is required to have amore highly-smoothed inner surface is subjected to a highly-smoothingtreatment such as electrochemical polishing after it is cold drawn bythe above-described method.

Typically, there are two types of methods for cold drawing in whichdifferent plugs are used as shown in FIG. 10.

FIG. 10(a) shows a method in which a cylindrical plug 81 whose outsidediameter is uniform is used. The cylindrical plug 81 is connected with aplug-supporting rod 87. This method is used for producing pipes ofrelatively large dimensions.

FIG. 10(b) shows a method in which a floating plug 82 is used. Thismethod is characterized by the shape of the plug and by the method forsupporting the plug. As illustrated in this figure, the floating plug 82is tapered, and the cone angle 2β of the plug is smaller than the facialangle 2α of the die.

For this reason, those forces which act on the floating plug 82 are thefrictional force which acts in the direction of drawing, and, inaddition to this, the pushing-back force which acts on the taperedsurface of the plug in the direction reverse to the direction ofdrawing. The frictional force and the pushing-back force are canceledand balanced with each other. Therefore, such a plug-supporting rod 87as is used in the method using a cylindrical plug shown in FIG. 10(a) isnot needed, and even if a supporting rod is provided in consideration ofoperation, almost no force acts on the supporting rod.

Since the floating plug has the above-described characteristics, themethod using this plug is commonly adopted to draw a tubing material toobtain, in particular, a pipe whose diameter is small. However, in thecase where this plug is used, the balancing position of the plug variesdepending upon the state of lubricating films provided on the inner andouter surfaces of the tubing material, or upon the force for drawing thetubing material. Since the change of the balancing position of the plugis extremely obstructive to the operation for drawing the tubingmaterial, proposals for improvements for this change have been made. Forinstance, Japanese Laid-Open Patent Publication No. 72419/1988 disclosesa method in which a plug is maintained at a proper position on a die bybalancing the frictional force and the pushing-back force which act,during the process of drawing, on the horizontal surface and the taperedsurface of the plug, respectively.

FIG. 11 is a longitudinal section explaining the shape of the plug whichis used for drawing a tubing material in the above-described method, andthe method of drawing. As illustrated in the figure, a plug 111 has ahorizontal surface 112 which provides a uniform outside diameter to theplug, a first tapered surface 113 with which the outside diameter of theplug is decreased toward the direction opposite to the direction ofdrawing, and a second tapered surface 114 which is continued to thefirst tapered surface and with which the outside diameter of the plug isincreased. Therefore, the forces which act on the horizontal surface 112and on the second tapered surface 114 when a tubing material is drawncan be balanced, and the plug 111 can thus be maintained at a properposition on the die 115.

The minimum inner surface roughness expressed in Rmax of a pipe obtainedby means of the above described conventional cold plug drawing islimited to approximately 1.1 micrometers, for example, when a clean pipehaving an outside diameter of 6 mm and a wall thickness of 1 mm isproduced by using SUS 304, and it is difficult to make the Rmax lowerthan this limit. Further, even in the method disclosed in JapaneseLaid-Open Patent Publication No. 72419/1988, it is necessary to make thedifference between the outside diameters on the inlet side and on theoutlet side of the part with the first tapered surface 113 asconsiderably large as several-tenths mm. When the difference between theoutside diameters of the part with the first tapered surface 113 islarge, seizure is caused when ordinary oil lubrication is conducted, sothat it is necessary to conduct chemical conversion treatmentlubrication which is excellent in antiseizure properties. However, aswill be described later, when chemical conversion treatment lubricationis conducted, the lubricating film produced is thick, so that a tubingmaterial after being subjected to drawing will have a high inner surfaceroughness. It is thus impossible to make the inner surface roughnessexpressed in Rmax to 1.0 micrometer or less even when squeezing isconducted. Therefore, in order to obtain a pipe which is required tohave an inner surface roughness expressed in Rmax of 1.0 micrometer orless, it is essential, as mentioned above, to conduct a highly-smoothingtreatment such as electrochemical polishing after cold plug drawing isconducted. As a result, the price of the final product becomes extremelyhigh, approximately four times the price of a pipe produced byconventional cold plug drawing.

Some techniques have been known as methods in which a plug of a specificshape is used when cold plug drawing is conducted in order to bringabout special effects on the inner surface of a pipe.

For instance, Japanese Patent Publication No. 7244/1987 discloses amethod in which a tubing material is processed by using a plug of aspecific shape to form a work-hardened layer on the inner surface of apipe so as to prevent the stainless steel pipe from being oxidized bywater vapor.

FIG. 12(a-c) include a side view and longitudinal sections which showthe shape of a plug having a protruding portion, used for theabove-described processing, and a method of drawing, using the plug.FIG. 12(a) and FIG. 12(b) are illustrations showing the process ofdrawing, and FIG. 12(c) is a side view of the plug. Shown in this figureis a method in which a work-hardened layer is formed on the innersurface of a tubing material 95 by increasing the inside diameter of thetubing material 95 by the use of plug 91 provided with a protrudingportion 94 and a plug-supporting rod 97 as illustrated in the figure.The object of this method is to form a work-hardened layer, and theinner surface roughness obtained, expressed in Rmax is extremely highfrom 18 to 25 micrometers. Further, in this method, the tubing material95 is processed without constraining the outer surface thereof, so thatit is impossible to obtain 1 micrometer or less of the inner surfaceroughness expressed in Rmax as in a part of the examples which will bedescribed later.

A method shown in FIG. 13 has also been used as another conventionalmethod of processing. FIG. 13 is a side view, partly in cross section,and a longitudinal section which show the shape of a conventional plugused for increasing the inside diameter of a pipe, and a method ofdrawing, using the plug. The object of this method is to improve thedimensional accuracy of the inner surface of a pipe such as a steel pipefor a cylinder (in particular, the roundness of the inside of a pipe).

As shown in FIG. 13, plug 101 connected with a supporting rod 107 is sotapered that the diameter of the plug is slightly increased toward theoutlet side of drawing. The inside diameter of a tubing material 105 isincreased by this plug 101 and a die 106, and, at the same time, theroundness of the inside of the tubing material 105 is improved. However,since the increase of the inside diameter is slightly in this method,the effect of squeezing the inner surface of the tubing material, whichwill be described later, is small, that is, only a thin layer of shearplastic deformation is formed on the inner surface of the tubingmaterial. It is therefore impossible to obtain a highly-smoothed innersurface which is required for a clean pipe.

As illustrated in the figure, the diameter of the plug 101 is slightlyincreased toward the outlet side. However, with respect to thewall-thickness-processed part of the tubing material 105, there is nogreat difference between it and that part of the tubing materialobtained by using the conventional plug as shown in FIG. 10(a) and FIG.10(b). Therefore, although the roundness of the inside of a pipe can beimproved by the plug of this shape, the inner surface roughness of apipe cannot be improved.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a stainless steel pipeof the bright annealing finish type, having a highly-smoothed innersurface with an inner surface roughness expressed in Rmax of 1.0micrometer or less, and a method for producing the same, by which astainless steel pipe of the bright annealing finish type, having ahighly-smoothed inner surface can be inexpensively produced withoutconducting any inner-surface-highly-smoothing treatment such aselectrochemical polishing after cold plug drawing is conducted.

The gist of the present invention is a stainless steel pipe of thebright annealing finish type, having a highly-smoothed inner surface,set forth in the following item (1), and a method for producing thesame, set forth in the following items (2) to (4):

(1) A stainless steel pipe of the bright annealing finish type, having ahighly-smoothed inner surface, characterized in that the inner surfaceroughness Rmax of the stainless steel pipe is 1.0 micrometer or less.

(2) A method for producing a stainless steel pipe of the brightannealing finish type, having a highly-smoothed inner surface with aninner surface roughness expressed in Rmax of 1.0 micrometer or less, inwhich after a pipe is obtained by cold drawing, using a die and acylindrical plug, it is subjected to bright annealing treatment toobtain a clean pipe, characterized in that the cold drawing is conductedby using a plug having a ring-like protruding portion provided on a partof the finishing part which is on the hinder part of the plug, with theslanting surface on the inlet side of the ring-like protruding portionkept within a bearing part (a finishing part with a horizontal surface)of the die, followed by conducting bright annealing treatment.

(3) A method for producing a stainless steel pipe of the brightannealing finish type, having a highly-smoothed inner surface with aninner surface roughness expressed in Rmax of 1.0 micrometer or less, inwhich after a pipe is obtained by cold drawing, using a die and afloating plug, it is subjected to bright annealing treatment to obtain aclean pipe, characterized in that the cold drawing is conducted by usinga plug having a non-tapered part and a ring-like protruding portioncontinued thereto on a finishing part which is on the hinder part of theplug, with the slanting surface on the inlet side of the ring-likeprotruding portion kept within a bearing part (a finishing part with ahorizontal surface) of the die, followed by conducting bright annealingtreatment.

(4) A method for producing a stainless steel pipe of the brightannealing finish type, having a highly-smoothed inner surface with aninner surface roughness expressed in Rmax of 1.0 micrometer or less, inwhich after a pipe is obtained by cold drawing, using a die and afloating plug, it is subjected to bright annealing treatment to obtain aclean pipe, characterized in that the cold drawing is conducted by usinga plug having a ring-like concaved portion on awall-thickness-processing part which is the tapered part of the plug,followed by conducting bright annealing treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a side view, partly in cross section, and FIG. 1(b) alongitudinal section which show the shape of a shoulder-type cylindricalplug having a protruding portion, and a method of drawing, using thecylindrical plug; and FIG. 2(a) is a side view, partly in cross section,and FIG. 2(b) is a longitudinal section which show the shape of ashoulder-type floating plug having a protruding portion, and a method ofdrawing, using the floating plug.

FIG. 3 is a graph showing a change in the inner surface roughness Rmaxin the course of drawing conducted by the use of the shoulder-typefloating plug having a protruding portion. Further,

FIG. 4 is a longitudinal section showing the state that the innersurface of a tubing material is detached from the shoulder-type floatingplug having a protruding portion when drawing is conducted by using thefloating plug.

FIG. 5(a) is a side view and FIG. 5(b) is a longitudinal section whichshow the shape of a floating plug having a concaved portion, and amethod of drawing, using the floating plug. FIG. 5(a) is a general view,and FIG. 5(b) is an enlarged view of the part A (the concaved portionand the wall-thickness-processing part).

FIG. 6 is a graph showing a change in the inner surface roughness Rmaxin the course of drawing conducted by the use of the floating plughaving a concaved portion.

FIG. 7 is a side view, partly in cross section, and a longitudinalsection which show the dimensions and the shape of the shoulder-typecylindrical plug having a protruding portion, used in the example, and amethod of drawing, using the cylindrical plug; and FIG. 8(a) is a sideview, partly in cross section, and

FIG. 8(b) a longitudinal section which show the dimensions and the shapeof the shoulder-type floating plug having a protruding portion, used inthe example, and a method of drawing, using the floating plug. On theother hand,

FIGS. 9(a-b) are each a side we showing the dimensions and the shape ofthe floating plug having portion, used in the example.

FIG. 9(a) is a general view, and FIG. 9(b) is an enlarged view of thepart A (the concaved portion the wall-thickness-processing part).

FIG. 10(a) is a side view, partly in cross section, and FIG. 10(b) is alongitudinal section which show the shape of a conventional plug, and amethod of drawing, using the plug. In this figure, FIG. 10(a) shows acylindrical plug, and FIG. 10(b) shows a floating plug. Further,

FIG. 11 is a longitudinal section explaining the shape of a floatingplug used for conventional drawing of tubing material, and a method ofdrawing, using the floating plug. Furthermore,

FIGS. 12(a-c) are a side view and longitudinal sections which show theshape of a conventional plug having a protruding portion, and a methodof drawing, using the plug. In this figure, FIG. 12(a) and FIG. 12(b)show the process of drawing, and FIG. 12(c) is a side view of the plug.

FIG. 13 is a side view, partly in cross section, and a longitudinalsection which show the shape of a conventional plug used for increasingthe inside diameter of a pipe, and a method of drawing, using the plug.

FIG. 14 is a table showing the results of the measurement of the innersurface roughness of pipes obtained by the use of the cylindrical plugout of the shoulder-type plugs having a protruding portion; and

FIG. 15 is a table showing the results of the measurement of the innersurface roughness of pipes obtained by the use of the floating plug.

FIG. 16 is a table showing the results of the examination on theeffects, on the inner surface roughness of a pipe obtained by using theshoulder-type floating plug having a protruding portion, of the angle γof the slanting surface of the protruding portion provided on the plug.

Further, FIG. 17 is a table showing the results of the examination onthe effects, on the inner surface roughness of a pipe obtained by usingthe shoulder-type cylindrical plug having a protruding portion or theshoulder-type floating plug having a protruding portion, of the lengthE₂ of the horizontal part of the protruding portion provided on theplug, and of a coating provided on the plug.

FIGS. 18 and 19 are tables showing the results of the measurement of theinner surface roughness of pipes obtained by using the floating plughaving a concaved portion, with the depth Δd of the concaved portion orE₃ changed. Further, FIG. 20 is a table showing the results of theexamination of the effects of the angle θ on the inner surface roughnessof a pipe obtained by the use of the floating plug having a concavedportion.

BEST MODE FOR CARRYING OUT THE INVENTION

As the methods for lowering the roughness on the surface of a processedmetal, a method has been known in which the roughness on the surface ofa tool is lowered, and a known method in which a lubricating oil capableof forming a thin lubricating film is used. The inventors of the presentinvention found that when the surface of a material to be processed,that is, the inner surface layer of a tubing material is squeezed, inaddition to applying the above known techniques, by using a tool havinga smooth surface (low roughness) to concentrate thereon shear plasticdeformation, the shear plastic deformation is concentrated on thesurface layer of the material to be processed, and the surface of thematerial fits the surface of the tool, whereby the surface roughness isstill lowered very much. The present invention has been accomplished onthe basis of this finding.

The object of the method of the present invention is to lower theroughness on the inner surface of a pipe by conducting cold drawingonly. To attain this object, it is necessary to conduct "squeezing" sothat shear plastic deformation can be concentrated on the inner surfacelayer of the pipe. The methods for conducting "squeezing" in the presentinvention include a method referred to as the first method of production(the method of production described in the previously-mentioned item (2)or (3)), in which a shoulder-type plug having a protruding portion isused, and a method referred to as the second method of production (themethod of production described in the previously-mentioned item (4)), inwhich a floating plug having a concaved portion is used.

The action and effects of the stainless steel pipe having ahighly-smoothed inner surface and the method for producing the sameaccording to the present invention will now be explained below.

1. Inner Surface Roughness of Stainless Steel Pipe of Bright AnnealingFinish Type, Having Highly-Smoothed Inner Surface

The reason why the inner surface roughness expressed in Rmax of thestainless steel pipe of the present invention, having a highly-smoothedinner surface is restricted to 1.0 micrometer or less will be explainedbelow.

The inner surface roughness expressed in Rmax of a pipe drawn by theconventional method is limited to approximately 1.1 micrometers, andcannot be drawn to 1.0 micrometer or less. For this reason, there hasbeen no such stainless steel pipe produced by means of cold drawing thathas an inner surface roughness expressed in Rmax of 1.0 micrometer orless. At the current level of technology, the surface roughness of aplug, expressed in Rmax is limited to approximately 0.1 micrometers.Theoretically, the inner surface roughness of a tubing material can beimproved to this degree, but it is, in practice, limited to 0.3micrometers. However, if the surface roughness of a plug, expressed inRmax can be made to 0.1 micrometers or less, the inner surface roughnessof a pipe can be drawn much lower. On the other hand, when the innersurface roughness of a pipe, expressed in Rmax is in excess of 1.0micrometer, the reduction of fine particles or impurities cannot beattained as desired.

2. First Method of Production (A case where a shoulder-type plug havinga protruding portion is used)

FIGS. 1(a-b) are a side view, partly in cross section, and alongitudinal section which show the shape of a shoulder-type cylindricalplug having a protruding portion, and a method of drawing, using thecylindrical plug.

In the first method of production according to the present invention, aring-like protruding portion (hereinafter referred to simply as aprotruding portion) 14 having an increased diameter is provided, in thecase of a cylindrical plug as shown in FIG. 1, on the hinder part of thecylindrical plug 11 connected with a plug-supporting rod 17, and theinner surface of a tubing material 15 is squeezed by this protrudingportion in order to concentrate shear plastic deformation on the innersurface of a pipe by conducting cold plug drawing. Namely, within thelength L of the finishing part with a horizontal surface (bearing part)19 of the die 16, a predetermined amount Δh of wall-thickness processingis further given by the protruding portion 14 provided on thecylindrical plug 11 after wall-thickness processing is conducted by adie 16 and the cylindrical plug 11.

At this moment in the process, it is the slanting surface 18 on theinlet side of the protruding portion 14 that is brought into contactwith the inner surface of the tubing material 15 to give the amount Δhof wall-thickness processing. When the amount Δh of wall-thicknessprocessing is given under such a state that the slanting surface 18 iswithin the length L of the finishing part with a horizontal surface(bearing part) 19 of the die 16 during the process of drawing, that is,under such a state that the outside diameter of the tubing material 15is constrained, shear plastic deformation is concentrated on the innersurface of the tubing material, whereby the effect of improving theinner surface roughness can be enhanced. As a result, it is possible tomake the inner surface roughness expressed in Rmax to 1.0 micrometer orless.

When the slanting surface 18 of the protruding portion 14 is in thebearing part 19, the outside diameter and the wall thickness of thetubing material 15 are determined by the die 16 and the cylindrical plug11 before the tubing material 15 is squeezed by the protruding portion14. Therefore, the amount Δh of wall-thickness processing becomesconstant without being affected by the uniformity of the wall thicknessand the roundness of the mother pipe, and the difference in the innersurface roughness after the process of drawing becomes small. Aftersqueezing is conducted (the position on the outlet side, posterior tothe slanting surface 18), the inside and outside diameters of the tubingmaterial 15 are kept constant, and wall-thickness processing is notconducted. Therefore, seizure is also not caused between the cylindricalplug 11 and the inner surface of the tubing material 15.

When the slanting surface 18 gets out of the bearing part 19, and goesto the outlet side, since the outer surface of the tubing material 15 isnot constrained, a part of the amount Δh of wall-thickness processing isabsorbed in the increased outside diameter of the tubing material 15. Asa result, the effect of improving the inner surface roughness isdrastically decreased as compared with the case where the outer surfaceof the tubing material 15 is constrained. It is thus impossible to makeRmax to 1.0 micrometer or less. Furthermore, when it is tried to enhancethe effect of improving the inner surface roughness by increasing theamount Δh of wall-thickness processing, seizure is caused between thecylindrical plug 11 and the inner surface of the tubing material 15.

When the slanting surface 18 gets out of the bearing part 19 and comesto the inlet side, the outside diameter of the tubing material 15 isreduced by the die 16 at a position on the outlet side, posterior to theslanting surface 18 after the tubing material 15 is squeezed by theslanting surface 18. The inside diameter is thus kept constant by plug11, whereby the material 15 undergoes wall-thickness processing. Whenthe squeezing is conducted by the slanting surface 18, the amount of alubricating oil existing between the surface of plug 11 and the innersurface of the tubing material 15 is decreased, and the thickness of thelubricating oil film becomes too thin. As a result, the oil film ispartially ineffective. Seizure is thus caused if the tubing material 15is subjected to wall-thickness processing after it is squeezed. For theabove-described reason, when the slanting surface 18 gets out of thebearing part 19 to the inlet side, it is impossible to make the innersurface roughness expressed in Rmax to 1.0 micrometer or less.

The above-described relative position of the protruding portion 14 tothe bearing part 19 of the die can be set up by adjusting the length ofthe plug-supporting rod 17 in the case of the cylindrical plug 11.

FIGS. 2(a-b) are a side view, partly in cross section, and alongitudinal section which show the shape of a shoulder-type floatingplug having a protruding portion, and a method of drawing, using thefloating plug.

In the case where a floating plug is used in the first method ofproduction according to the present invention, a protruding portion 14which has, on the inlet side, a slanting surface 18 with an increaseddiameter is continuously provided next to the non-tapered part 13n ofthe finishing part 13 of the floating plug 12 as shown in FIG. 2 so asto squeeze the inner surface of a tubing material 15. Namely, afterwall-thickness processing is conducted by the die 16 and the non-taperedpart 13n of the finishing part 13, a predetermined amount Δh ofwall-thickness processing is further produced by the die 16 and theprotruding portion 14 of the floating plug 12 at the slanting surface 18on the inlet side of the protruding portion 14 provided on the finishingpart 13.

Also in this case, the amount Δh of wall-thickness processing isproduced under such a state that the slanting surface 18 on the inletside of the protruding portion 14 is within the length L of thefinishing part with a horizontal surface (bearing part) 19 of the die 16during the process of drawing, that is, under a state that the outersurface of the tubing material 15 is constrained.

The reason for the above-described restriction imposed in the methodusing the floating plug is the same as that in the previously-mentionedcase where the cylindrical plug is used.

In the course of drawing the tubing material, the floating plug 12floats so that the point "a" shown in FIG. 2(b), which is the turningpoint from the tapered part of the plug to the horizontal part(non-tapered part) of the finishing part 13, and the point "b" shown inFIG. 2(b), which is the turning point from the tapered part to thebearing part 19 of the die 16, can almost line up with each other on theaxial direction. When the distance E₁ between the inlet-side end of thebearing part L of the die and the point at which the horizontal part ofthe protruding portion 14 begins is in excess of the length L of thebearing part of the die 16, the slanting surface 18 on the inlet side ofthe protruding portion 14 gets out of the bearing part 19. It istherefore necessary to select the dimensions of the floating plug 12 andthe die 16 so that the relation between E₁ and L can be E₁ ≦L.

It is the slanting surface 18, which is anterior to the horizontal partE₂ of the protruding portion 14, that practically squeezes the innersurface of the tubing material 15. Therefore, the length E₂ of thehorizontal part of the protruding portion does not basically affect theinner surface roughness of the tubing material after being subjected todrawing. However, when the length of the horizontal part is too long,seizure is caused between this part and the inner surface of the tubingmaterial. The desirable length E₂ of the horizontal part of theprotruding portion is less than 3.0 mm.

The action and effects obtainable when a floating plug is used in thefirst method of production according to the present invention will beexplained by referring to FIG. 3.

FIG. 3 is a graph showing a change in the inner surface roughness Rmaxin the course of drawing conducted by using the shoulder-type floatingplug having a protruding portion. Point B is a position at which thetubing material comes into contact with the die, point C is a positionat which the inner surface of the pipe comes into contact with the plug,and point D is a position at which the tapered part of the plug isturned to the non-tapered part which is the finishing part. As shown inthis graph, the inner surface roughness of the pipe is increased in theregion of sink drawing (B→C), which is before the inner surface of thepipe comes into contact with the plug. It is decreased in the region ofwall-thickness processing (C→D), which is after the inner surface of thepipe comes into contact with the plug, and further decreased by theslanting surface of the protruding portion after processing is conductedat the non-tapered part. After the above processing, an Rmax ofapproximately 0.5 micrometers can be obtained satisfactorily. The term"ordinary plug" described in FIG. 3 means a plug having no protrudingportion (Δh=0), and a change in Rmax in the case where such an ordinaryplug is used is shown by a dotted line. Namely, the graph shows a changein Rmax until processing is conducted at the non-tapered part which isthe finishing part.

The effect of the height of the protruding portion, that is, the amountΔh of wall-thickness processing is such that the amount of squeezing isincreased as Δh is increased, and the inner surface roughness of thetubing material after subjected to drawing also becomes lower. However,when Δh is too large, the surface layer is severely processed, so thatseizure tends to be caused. Moreover, the inner surface of the tubingmaterial is detached from the surface of the plug during the processing,and the amount of squeezing becomes smaller than Δh. The desirable rangeof Δh is from 0.01 to 0.08 mm.

FIG. 4 is a longitudinal section showing the state at the time when theinner surface of the tubing material is detached from the plug in thecase where drawing is conducted by using the shoulder-type floating plughaving a protruding portion within die 36. As illustrated in thisfigure, when Δh is made too large, the inner surface of the tubingmaterial 35 is detached from tapered and non-tapered surfaces of thefloating plug 32 during the processing, and the amount of Δh' ofwall-thickness processing conducted by squeezing becomes smaller thanthe practical height Δh of the protruding portion. The effect ofimproving the inner surface roughness is thus determined by Δh', so thatit cannot be enhanced even if Δh is made larger than the upper limit ofthe above-described range.

For this reason, it is preferable that the height of the protrudingportion, that is, the amount Δh of wall-thickness processing be madeequal to or less than the critical value at which the detachment of theinner surface of the tubing material from the surface of the plug beginsto occur.

Even in the case where a predetermined amount Δh of wall-thicknessprocessing is given to the inner surface of the tubing material, theeffect of the angler formed with the slanting surface of the protrudingportion and the non-tapered part which is the finishing part should betaken into consideration. When the angle γ of the slanting surface istoo small, wall-thickness processing gently proceeds during the processof squeezing, so that the squeezing effect cannot be fully obtained. Itis thus impossible to obtain an Rmax of 1.0 micrometer or less. On theother hand, when the angle γ of the slanting surface is too large,wall-thickness processing proceeds drastically, so that seizure tends tobe caused on the slanting surface. The desirable range of the angle γ ofthe slanting surface is from 10° to 50° as shown in the examples, whichwill be described later.

3. Second Method of Production (A case where a floating plug having aconcaved portion is used)

FIGS. 5(a-b) are a side view and a longitudinal section which show theshape of a floating plug having a concaved portion, and a method ofdrawing, using the floating plug. FIG. 5(a) is a general view, and FIG.5(b) is an enlarged view of the part A (the concaved portion and thewall-thickness-processing part).

As illustrated in this figure, even in the case where a concaved portion42 is provided on the inlet side of the wall-thickness-processing partwhich is the tapered part of a floating plug 41, that is, an approachpart, the inner surface of a tubing material 45 is squeezed at asqueezing point 43 which is on the edge on the outlet side of theconcaved portion 42 and a die 46. At this moment in the process, thesame effect is obtained as in the case where a plug provided with aprotruding portion is used, and the roughness on the inner surface ofthe tubing material becomes lower. As a result, it becomes possible toobtain an inner surface roughness expressed in Rmax of 1.0 micrometer orless. It is noted that Δd shown in FIG. 5(b) is called the depth of theconcaved portion for convenience. Δh represents the amount ofwall-thickness processing (the height of the protruding portion), and θrepresents the angle formed with the contour of the plug 41 in thevicinity of the squeezing point 43 which is on the edge on the outletside of the concaved portion 42, and the tapered surface of the plug.

The effect of this concaved portion 42 on the improvement of the innersurface roughness of the tubing material is enhanced by squeezing as theinner surface of the tubing material 45 gets nearer the bearing part(horizontal part) 49 of the plug.

More specifically, it is necessary to provide this concaved portion 42on the outlet side posterior to the point (C shown in FIG. 6, which willbe described later) at which the inner surface of the tubing material 45begins to touch with the floating plug 41. Namely, it is necessary thatthe concaved portion 42 be entirely within the region of wall-thicknessprocessing (C to D shown in FIG. 6, which will be described later) inwhich the tapered part of the plug comes into contact with the innersurface of the tubing material 45.

The reason for the above is as follows. Before the inner surface of thetubing material 45 comes into contact with the plug 41 (B to C shown inFIG. 6, which will be described later), the inner surface roughness ishigh. Although the inner surface roughness is lowered when the tubingmaterial is squeezed at the squeezing point 43 (F shown in FIG. 6, whichwill be described later) which is on the edge on the outlet side of theconcaved portion 42, it is impossible to obtain a Rmax of 1.0micrometer. After the squeezing is conducted, the shape of the plughaving this concaved portion 42 is the same as that of an ordinaryfloating plug. Therefore, it is also impossible to make the innersurface roughness expressed in Rmax to 1.0 micrometer or less. Itbecomes possible to make the inner surface roughness expressed in Rmaxto 1.0 micrometer or less by squeezing the inner surface of a tubingmaterial after the roughness thereof is once made lower by bringing theinner surface of the tubing material 45 into contact with the plug 41.

Upon the determination of E₃ shown in FIGS. 5(a-b) (the distance betweenthe outlet-side end of the tapered part of the plug and the squeezingpoint which is on the edge on the outlet side of the concaved portion),drawing is conducted by using the same mother pipe, die and lubricatingoil, and an ordinary floating plug of the same dimensions, having noconcaved portion. In this process, the processing is suspended beforethe tubing material is completely drawn out from the die, and theportion under processing is broken in half, thereby confirming theposition at which the inner surface of the tubing material begins tocome into contact with the plug, and the region of wall-thicknessprocessing. The position of E₃ is thus determined so that the concavedportion can entirely be within this region.

Specifically, when the length of the region of wall-thickness processingwhich is on the tapered part of the plug is referred to as C_(L), it isdesirable that the range of E₃ fulfill the following inequality (1):

    E.sub.3 <[C.sub.L Δd/tan β]                     (1)

A smaller E₃ brings about a greater effect in improving the innersurface roughness as long as E₃ fulfills the above-described condition.Therefore, it is desirable that E₃ fulfill the following inequality (2):

    0≦E.sub.3 <[C.sub.L Δd/tan β]            (2)

FIG. 6 is a graph showing a change in the inner surface roughness Rmaxin the case where drawing is conducted by using the floating plug havinga concaved portion. Point B is the position at which the tubing materialcomes into contact with the die, point C is the position at which thetubing material comes into contact with the plug, point D is theposition at which the tapered part of the plug is turned to thehorizontal part, and point F is the position at which the tubingmaterial is squeezed by the edge on the outlet side of the concavedportion. The inner surface roughness of the pipe is increased in theregion of sink drawing (B→C), which is before the tubing material comesinto contact with the die. It is decreased in the region ofwall-thickness processing (C→F→D), which is after the tubing materialcomes into contact with the plug, and drastically decreased when thetubing material is squeezed at the point F between C and D. After theprocessing, a Rmax of approximately 0.5 micrometers can be obtainedsatisfactorily. The term "ordinary plug" shown in FIG. 6 means a plughaving no concaved portion (Δd=0), and a change in Rmax in the casewhere such an ordinary plug is used is shown by a dotted line.

With respect to the shape of the concaved portion 42, when the depth Δdof the concaved portion is increased, the amount Δh of wall-thicknessprocessing given by squeezing is increased, and the effect of improvingthe inner surface roughness is enhanced. However, a free surface is alsoincreased along with the increase of Δd, so that the inner surfaceroughness on such a surface becomes high. As a result, the effect ofimproving the inner surface roughness obtainable by squeezing isdecreased. Namely, when Δd is too large, the effect of improving theinner surface roughness is canceled, and seizure tends to be causedbetween the inner surface of the tubing material and the plug.

However, it is not Δd but the amount Δh of wall-thickness processinggiven by squeezing that determines the condition for the occurrence ofseizure. Δh is determined by the facial angle 2α of the die, the angle2β of a taper provided on the plug and Δd, and geometrically obtained bythe following equation (3):

    Δh=[Δd·sin β·(tan α-tan β)]/(tan α·tan β)                (3)

The desirable range of Δh is from 0.01 to 0.08 mm as in the case of theshoulder-type floating plug having a protruding portion.

When the angle θ shown in FIG. 5(b) is too small, the wall-thicknessprocessing proceeds slowly by squeezing, so that the effect of improvingthe inner surface roughness becomes small. It is therefore impossible toobtain a Rmax of 1.0 micrometer or less. On the other hand, when theangle θ is too large, the wall-thickness processing proceedsdrastically, so that seizure is caused between the inner surface of thetubing material and the plug. The desirable range of θ is from 10° to50°.

4. Materials for Tools, Lubricant, Etc.

When shear plastic deformation is concentrated on the inner surface of apipe by squeezing as in the method (the first and second methods ofproduction) of the present invention, a work-hardened layer is formed,and crystal grains are finely divided. In the case of stainless steelpipes, the corrosion resistance is also improved when crystal grains arefinely divided.

It is better to make the surface roughness of the plug lower than therequired inner surface roughness of a tubing material. When the surfaceroughness of the plug is higher than the desired inner surfaceroughness, the inner surface roughness of the tubing material after theprocess of drawing also becomes high. It is therefore desirable that theroughness on the surface of the plug be made as low as possible.

Those materials which have high hardness, such as sintered hard alloysare favorable as the materials for the die and the plug. In the casewhere the tubing material is a material which readily undergoes seizure,it is desirable to coat the surface of the die and that of the plug witha material which is excellent in antiseizure properties, such as TiCN.

As a lubricant for use in the process of drawing, it is desirable to useone which can form a thin lubricating film, for example, a mixed oil ofsulfurized oil and chlorinated paraffin.

After the process of cold drawing, removal of lubricant is conducted,and bright annealing treatment is then carried out in the conventionalmanner. When the bright annealing treatment is carried out in anoxidizing atmosphere, scale is produced, and the inner surface roughnessof the tubing material becomes high. Moreover, the scale deposited onthe inner surface becomes particles, so that the properties required fora clean pipe are impaired. In order to prevent the above, brightannealing is conducted by the use of a hydrogen furnace or vacuumheating furnace where scale is not produced.

Thereafter, a treatment of bend straightening is carried out, whennecessary. However, in the case of the stainless steel pipe of thebright annealing finish type, having a highly-smoothed inner surfacewhich the present invention does, it is not necessary to carry out ahighly-smoothing treatment such as electrolytic polishing, whichincreases the production cost. In the case where a pipe having a morehighly-smoothed inner surface when Rmax is less than 0.3 micrometers isrequired depending upon the use thereof, it is necessary to conductelectrochemical polishing or a like method, but the time for polishingcan be drastically shortened.

The action and effects of the method of the present invention will nowbe explained by referring to the following specific examples.

1. A case where a shoulder-type plug having a protruding portion is used

Mother pipes made of SUS316L, having an outside diameter of 11 mm, awall thickness of 1.3 mm and an inner surface roughness expressed inRmax of 1.8 micrometers ware subjected to cold drawing, using two typesof plugs, a cylindrical plug and a floating plug, thereby obtainingpipes having an outside diameter of 8.4 mm and a wall thickness of 1.2mm.

First, a case where a cylindrical plug was used will be explained. Theshape and dimensions of the cylindrical plug used are shown in FIG. 7.The height Δh of the protruding portion has a range of 0.005 to 0.10 mm.

A sintered hard alloy (equivalent to JIS V20) having a surface roughnessexpressed in Rmax of 0.1 micrometers was used as the material for thedie and the plug.

Oil lubrication was adopted for lubrication, and the cold drawing wasconducted with a lubricating oil (a mixed oil of sulfurized oil andchlorinated paraffin) applied to the inner and outer surfaces of thepipe.

The cold drawing was conducted by adjusting the length of theplug-supporting rod so that the distance 1, shown in FIG. 7, between theinlet-side end of the bearing part L (L: 1.0 mm) of the die and thepoint at which the horizontal part of the protruding portion beginswould be changed from 0 to 3.0 mm, and Rmax was determined from theinner surface roughness. The above-described conditions and the resultsare shown in FIG. 14.

As shown in FIG. 14, when the position at which the horizontal part ofthe protruding portion begins is outside the bearing part of the die,having the length L, that is, when the protruding portion gets out ofthe bearing part of the die to the outlet side, the effect of improvingthe inner surface roughness is decreased, and an Rmax of 1.0 micrometeror less cannot be obtained. When l is 1.0 mm or less, that is, when theposition at which the horizontal part of the protruding portion beginsis inside the bearing part of the die, Rmax can obtain stability, and isnot greatly changed by the change of l.

With respect to the height Δh of the protruding portion, 0.005 mm of Δhis too small, and the effect of improving the inner surface roughnessobtainable by squeezing is small. It is thus impossible to obtain anRmax of 1.0 micrometer or less. On the other hand, when Δh is asextremely large as 0.10 mm, seizure was caused. It can thus be knownthat the preferable range of Δh is from 0.01 to 0.08 mm.

Next, a case where a floating plug is used will be explained. The shapeand dimensions of the floating plug used are shown by the general viewand the enlarged view of part A in FIGS. 8(a-b). Also in this case, theheight Δh of the protruding portion has a range of 0.005 to 0.10 mm.Further, the material for the die and the plug, the surface roughnessthereof, and the conditions for the lubricating oil were made the sameas those in the case where the cylindrical plug was used.

Cold drawing was conducted under such a condition that the distance E,between the inlet-side end of the bearing part L (L: 1.0 mm) of the dieand the position at which the horizontal part of the protruding portionbegins was changed from 0.25 to 3.0 mm, and Rmax was determined as theinner surface roughness. The above described conditions and the resultsare shown in FIG. 15.

As shown in FIG. 15, 0.005 mm is too small as the height Δh of theprotruding portion, so that the effect of improving the inner surfaceroughness cannot be obtained as in the case where the cylindrical plugis used. It is thus impossible to obtain an Rmax of 1.0 micrometer orless. On the other hand, when the height Δh was 0.1 mm, seizure wascaused. It can thus be proven that the desirable range of the height Δhis from 0.01 to 0.08 mm.

When E₁ is in excess of 1.0 mm, that is, when it is longer than thelength L of the bearing part of the die, the protruding portion gets outside of the bearing part of the die to the outlet side of the die, sothat the effect of improving the inner surface roughness is decreased.It is therefore impossible to obtain a Rmax of 1.0 micrometer or less.When E₁ is equal to or shorter than the length of the bearing part ofthe die, that is, when the protruding portion is inside the bearing partof the die, the effect of improving Rmax can be obtained satisfactorily.Therefore, it can be proven that it is desirable to control E₁ to beequal to or shorter than L.

Thus, it is possible to make the inner surface roughness of a tubingmaterial after subjected to drawing expressed in Rmax to 1.0 micrometeror less by using either the shoulder-type cylindrical plug having aprotruding portion or the shoulder-type floating plug having aprotruding portion. The inner surface roughness of the tubing materialremains unchanged even if bright annealing treatment is then carried outin accordance with the ordinary method. A clean pipe of the BA type,having the above-described inner surface roughness Rmax can thus beproduced.

The surface roughness expressed in Rmax of the plug used in this examplewas made to 0.1 micrometers, which can be obtained by the presenttechnology. The inner surface roughness of the tubing material aftersubjected to drawing was slightly higher than the surface roughness ofthe plug, but nearly equal to the level of the surface roughness of theplug. If a plug having a lower surface roughness is used, it is possibleto attain a further reduction in the inner surface roughness of thetubing material.

Further, with respect to the shoulder-type floating plug having aprotruding portion, the effect on the improvement on the inner surfaceroughness of a tubing material, of the angler of the slanting surface ofthe protruding portion provided on the plug, was examined.

The basic shape and dimensions of the plug and the die used are the sameas those shown in FIG. 8. The inner surface roughness was measured underthe following conditions: the height Δh of slanting of the protrudingportion was 0.04 mm, E₁ E₂ was 0.5 mm, and the angle γ was changed from5° to 60°. The results are shown in FIG. 16.

As is clear from FIG. 16, when the angle γ was 5°, Rmax was 1.3micrometers, and the squeezing effect obtained was small; and when theangle γ was 60°, the wall-thickness processing proceeded drastically, sothat seizure was caused on the slanting surface of the protrudingportion. It is therefore desirable to control the angler within therange of 10° to 50°. The same examination was made also on theshoulder-type cylindrical plug having a protruding portion, and it wasconfirmed that the results obtained were the same as the above.

Next, with respect to the shoulder-type cylindrical plug having aprotruding portion and the shoulder-type floating plug having aprotruding portion, tests were carried out in order to examine theeffects on the inner surface roughness of a tubing material, of thelength E₂ of the horizontal part of the protruding portion provided onthe plug, and of the coating provided on the plug.

The basic shape and dimensions of the plug and the die used are the sameas those shown in FIGS. 7 and 8. The processing conditions were the sameas those for the above-described test. E₂ was changed from 0.5 to 3.0 mmin both cases using the cylindrical plug and the floating plug, and Δhand E₁ were kept constant to 0.04 mm and 0.5 mm, respectively. The aboveconditions and the results are shown in FIG. 17.

As shown in FIG. 17, the length E₂ of the horizontal part of theprotruding portion did not affect the inner surface roughness of thetubing material after subjected to drawing. However, when E₂ was 3.0 mm,seizure was caused in both cases. It can thus be proven that thedesirable length E₂ of the horizontal part of the protruding portion isless than 3.0 mm.

Furthermore, a plug with a TiCN coating which has a low affinity for thetubing material, was applied in order to prevent seizure was subjectedto the test. Seizure was not caused even under the conditions shown inFIGS. 14-17 under which seizure was caused on the plugs that wereprovided with no coating.

2. A case where a floating plug having a concaved portion is used

Also in the case of a floating plug having a concaved portion, the samemother tube, die and lubricating oil as those used in the abovedescribed cases of the shoulder-type plug having a protruding portionwas used. The dimensions and shape of the plug used is shown in FIGS.9(a-b).

The plug used was one made of a sintered hard alloy (equivalent to JISV20), provided with a TiCN coating on the surface thereof, having asurface roughness expressed in Rmax of 0.3 micrometers.

Cold drawing was conducted by changing the depth Δd of the concavedportion from 0 to 0.30 mm (from 0 to 0.05 mm when converted in to Δh,provided that E₃ shown in FIG. 9 was kept constant to 0.5 mm), or bychanging E3 from 0 to 2.5 mm (provided that Δd was kept constant to 0.20mm, which was 0.05 mm when converted into Δh), and Rmax was determinedfrom the inner surface roughness. The results are shown in FIGS. 18 and19. It is noted that the angle θ shown in FIG. 9(b) was kept constant to10° in the process of drawing shown in FIGS. 18 and 19.

As shown in FIG. 18, the inner surface roughness can be obtainedsatisfactorily when Δd was 0.1 mm or more (0.01 mm or more whenconverted into Δh), and seizure was not caused. When compared with anordinary plug having no concaved portion, the effect of improving theinner surface roughness can be obtained in the case where the plughaving a concaved portion at a proper position thereon is used. When Δdis increased, the inner surface roughness of the tubing material tendsto be lowered.

In this case, C_(L) which is the length on the axial direction of theregion of wall-thickness processing which is on the tapered part of theplug is 2.8 mm. Further, Δd is 0.20 mm and the angle β is 10.5°.Therefore, the distance E₃, where the position of the concaved portionis determined, is 1.72 mm when obtained from the previously-mentionedinequality (1).

As shown in FIG. 19, when E₃ exceeds 1.72 mm and becomes 2.0 mm or 2.5mm, the inner surface roughness cannot be improved, and it is impossibleto obtain an Rmax of 1.0 micrometer or less. When E₃ is 1.72 mm or less,the effect of improving the inner surface roughness is enhanced, and theinner surface roughness becomes lower as E₃ becomes small.

Subsequently, the effect of the angle θ shown in FIG. 9(b) was examined.Cold drawing was conducted under the processing conditions which werebasically the same as those shown in FIGS. 18 and 19, provided that Δdand E₃ were kept constant to 0.15 mm and 0.5 mm, respectively, and theangle θ was changed from 0° to 60°. The test was determine whetherseizure was caused or not caused, and the inner surface roughness wasmeasured. The results are shown in FIG. 20.

As shown in FIG. 20, when θ was 0°, the shape of the plug was the sameas that of an ordinary plug, and the wall-thickness processing proceededslowly. Therefore, the inner surface roughness expressed in Rmax couldnot reach to 1.0 micrometer or less. When θ was 60°, the wall-thicknessprocessing proceeded drastically, so that seizure was caused.

As described above, in the case where the shoulder-type plug having aprotruding portion, and also in the case where the floating plug havinga concaved portion, a lubricating oil (a mixed oil of sulfurized oil andchlorinated paraffin) was used for the lubrication of the mother pipe.In order to compare with this, cold drawing is conducted under the sameconditions as those in the above, provided that chemical conversiontreatment lubrication (ferrous oxalate film) which is usually employedin the drawing of a stainless steel pipe was used. However, the objectwhich is to make the inner surface roughness of a tubing material,expressed in Rmax to 1.0 micrometer or less was not able to be attained.Therefore, in the practice of the method according to the presentinvention, chemical conversion treatment lubrication is not suitable.

The lowest limit of the inner surface roughness of a tubing material,expressed in Rmax is 0.3 micrometers in the above examples. This isbecause the the surface roughness of a plug, expressed in Rmax, at thecurrent level of technology is limited to approximately 0.1 micrometers.In addition, the thickness of a lubricating oil film also affects theinner surface roughness of a tubing material. Therefore, if the surfaceroughness of a plug, expressed in Rmax can be made to 0.1 micrometers orless, the inner surface roughness of a pipe can also be made smallerthan the above limit.

INDUSTRIAL APPLICABILITY

According to the present invention, a stainless steel pipe of the brightannealing finish type, having a highly-smoothed inner surface with aninner surface roughness expressed in Rmax of 1.0 micrometer or less,suitable for use in apparatus for the production of semiconductors, canbe obtained by conducting cold drawing and bright annealing treatmentonly. By this method, such a treatment that has been considered to beessential, such as electrochemical polishing, can be omitted. Theproduction cost can thus be greatly reduced. For this reason, thestainless steel pipe of the bright annealing finish type, having ahighly-smoothed inner surface according to the present invention can bewidely utilized in the fields of the production of semiconductors andthe like.

What is claimed is:
 1. A stainless steel pipe of the bright annealingfinish type, having a highly-smoothed inner surface, characterized inthat the inner surface roughness expressed in Rmax of the stainlesssteel is 1.0 micrometer or less, the inner surface roughness of 1.0micrometer or less being achieved by cold plug drawing withoutsubjecting the inner surface to electrochemical polishing after the coldplug drawing.
 2. The stainless steel pipe of claim 1, wherein the innersurface roughness is obtained by subjecting the inner surface to shearplastic deformation concentrated on the inner surface and forming a workhardened layer on the inner surface.
 3. The stainless steel pipe ofclaim 1, wherein the inner surface roughness is obtained by plug drawingduring which the exterior of the pipe passes through a die and theinterior of the pipe is work hardened by a plug, the surfaces of the dieand plug which contact the pipe being coated with TiCN or the innersurface roughness is obtained by plug drawing during which a lubricatingfilm of sulfurized oil and chlorinated paraffin is used.
 4. Thestainless steel pipe of claim 1, wherein the pipe has been subjected tobright annealing in a hydrogen furnace or vacuum heating furnace.
 5. Thestainless steel pipe of claim 1, wherein the inner surface roughness is0.3 to 0.5 micrometers.
 6. A method for producing a stainless steel pipeof the bright annealing finish type, having a highly-smoothed innersurface with an inner surface roughness expressed in Rmax of 1.0micrometer or less, in which after a pipe is obtained by cold drawing,using a die and a cylindrical plug, it is subjected to bright annealingtreatment to obtain a clean pipe, characterized in that the cold drawingis conducted by using a plug having a ring-like protruding portionprovided on a part of the finishing part which is on the hinder part ofthe plug, with the slanting surface on the inlet side of the ring-likeprotruding portion kept within a bearing part of the die, followed byconducting bright annealing treatment.
 7. The method of claim 6, whereinthe plug is supported by a rod or the plug is a floating plug, theprotruding portion of the plug being located within the die andplastically deforming the inner surface of the pipe by an amount in thedirection of the wall thickness of the pipe of 0.01 to 0.08 mm.
 8. Themethod of claim 6, wherein the surfaces of the die and plug whichcontact the pipe are coated with TiCN or a film of sulfurized oil andchlorinated paraffin is used as a lubricant on the die or plug.
 9. Themethod of claim 6, wherein the bright annealing is carried out in ahydrogen furnace or a vacuum heating furnace.
 10. A method for producinga stainless steel pipe of the bright annealing finish type, having ahighly-smoothed inner surface with an inner surface roughness expressedin Rmax of 1.0 micrometer or less, in which after a pipe is obtained bycold drawing, using a die and a floating plug, it is subjected to brightannealing treatment to obtain a clean pipe, characterized in that thecold drawing is conducted by using a plug having a non-tapered part anda ring-like protruding portion continued thereto on a finishing partwhich is on the hinder part of the plug, with the slanting surface onthe inlet side of the ring-like protruding portion kept within a bearingpart of the die, followed by conducting bright annealing treatment. 11.The method of claim 10, wherein the protruding portion of the plugcauses the inner surface of the pipe to undergo plastic deformation byan amount in the direction of the wall thickness of the pipe of 0.01 to0.08 mm.
 12. The method of claim 10, wherein the surfaces of the die andplug which contact the pipe are coated with TiCN or a lubricating filmof sulfurized oil and chlorinated paraffin is used as a lubricant duringthe cold drawing.
 13. The method of claim 10, wherein the brightannealing is carried out in a hydrogen furnace or a vacuum heatingfurnace.
 14. A method for producing a stainless steel pipe of the brightannealing finish type, having a highly-smoothed inner surface with aninner surface roughness expressed in Rmax of 1.0 micrometer or less, inwhich after a pipe is obtained by cold drawing, using a die and afloating plug, it is subjected to bright annealing treatment to obtain aclean pipe, characterized in that the cold drawing is conducted by usinga plug having a ring-like concaved portion provided on awall-thickness-processing part which is the tapered part of the plug,followed by conducting bright annealing treatment.
 15. The method ofclaim 14, wherein the concaved portion of the plug causes the innersurface of the pipe to undergo plastic deformation by an amount in thedirection of the wall thickness of the pipe of 0.01 to 0.08 mm.
 16. Themethod of claim 14, wherein the surfaces of the die and plug whichcontact the pipe are coated with TiCN or a lubricating film ofsulfurized oil and chlorinated paraffin is used as a lubricant duringthe cold drawing.
 17. A method for producing a stainless steel pipe ofthe bright annealing finish type, having a highly-smoothed inner surfacewith an inner surface roughness expressed in Rmax of 1.0 micrometer orless, in which after a pipe is obtained by cold drawing, using a die anda cylindrical plug, it is subjected to bright annealing treatment toobtain a clean pipe, characterized in that the cold drawing is conductedby using a plug having a ring-like protruding portion provided on a partof the finishing part which is on the hinder part of the plug, with theslanting surface on the inlet side of the ring-like protruding portionkept within a bearing part of the die, followed by conducting brightannealing treatment, the slanting surface forming an angle of 10 to 50°with the central axis of the plug.
 18. A method for producing astainless steel pipe of the bright annealing finish type, having ahighly-smoothed inner surface with an inner surface roughness expressedin Rmax of 1.0 micrometer or less, in which after a pipe is obtained bycold drawing, using a die and a floating plug, it is subjected to brightannealing treatment to obtain a clean pipe, characterized in that thecold drawing is conducted by using a plug having a non-tapered part anda ring-like protruding portion continued thereto on a finishing partwhich is on the hinder part of the plug, with the slanting surface onthe inlet side of the ring-like protruding portion kept within a bearingpart of the die, followed by conducting bright annealing treatment, theslanting surface forming an angle of 10 to 50° with the central axis ofthe plug.
 19. A method for producing a stainless steel pipe of thebright annealing finish type, having a highly-smoothed inner surfacewith an inner surface roughness expressed in Rmax of 1.0 micrometer orless, in which after a pipe is obtained by cold drawing, using a die anda floating plug, it is subjected to bright annealing treatment to obtaina clean pipe, characterized in that the cold drawing is conducted byusing a plug having a ring-like concaved portion provided on awall-thickness-processing part which is the tapered part of the plug,followed by conducting bright annealing treatment, the tapered part ofthe plug including a downstream portion located between the concavedportion and a non-tapered portion, the downstream portion of the taperedpart of the plug having a length of 1.72 mm or less.